The present invention concerns a method for setting and fixing jewels in parts of a watch movement, allowing both the relative positioning accuracy of the wheels to be increased and the cost price of the finished product to be reduced.
In high quality watch movements, in order to reduce friction and increase the longevity of the product, jewels or rubies are used essentially for the shoulders of the staffs of certain wheels, designated bearings, end-stones or cup-bearings, and as regards the escapement, for the entry and exit pallets of the lever escapement and for the impulse-pin. There currently exist different techniques for fixing these jewels in the appropriate places, such as the plate, bridge, escapement lever and roller plate, but the most common, it may be said since the beginning of the horological industry, consists in driving the jewel either into a setting, or more often directly into a housing machined in the part intended to receive said jewel. Tolerances both as regards the jewel and the housing have to be very low, of the order of 5xcexc, in order to prevent the jewel deforming the housing or conversely having any play during use. For bearings, it is also imperative to have perfect vertical positioning of the drilled hole, which is not necessarily obtained by driving in the jewels given that they generally have rounded edges. Likewise, precise positioning along the vertical axis is also necessary so that two facing bearings, or two facing end-stones respect the distance-of-centres provided for the wheels which they support, which requires precise machining both of the thickness of the jewel and of the bottom of the housing. Moreover, this requirement for accuracy is also necessary for the relative positioning of the drilled holes for the bearings arranged in a same plane and intended to receive gear-wheel pivots. This spatial positioning is also of very high importance as regards the escapement jewels, impulse pin and pallets, which are currently driven in and/or bonded. Driving in jewels inevitably causes a deformation in the roller-table which then has to be rectified by diamond polishing and bonding also necessitates a machining operation in order to remove the surplus of the adhesive material used.
In order not to be tied to such a high level of accuracy and to avoid the drawbacks of driving in the jewels, Swiss Patent No. 384 473 proposes a cam device allowing an end-stone to be wedged in two hollows in the wall of the housing. The device disclosed in Swiss Patent No. 362 286 proposes a similar device formed by a bayonet assembly device. Other devices propose means for wedging a jewel via elastic rings locking into grooves in the wall of the recess. It is clear that such devices permit lower tolerance requirements as regards machining the jewel and the housing, but they are not entirely satisfactory as regards the spatial positioning of the jewels and securing them to their support.
As regards the machines or robots which set and secure the jewels, to the best of the Applicant""s knowledge, these are driving techniques such as those disclosed for example in Swiss Patent Nos. 378 242 and 417 479.
The object of the present invention is to overcome the drawbacks of the aforecited prior art by transferring the requirement for accuracy to a setting device and relying on laser technology to secure the jewels in their housing.
The invention thus concerns a method allowing jewels to be secured in parts of a watch movement by laser beam by means of a setting device. xe2x80x9cPartsxe2x80x9d means the fixed parts of a watch movement, such as the plates and bridges, or the moving parts such as the escapement, the jewels then being designated bearings, end-stones, impulse pins or pallet-stones. These jewels are obtained synthetically, from a fine alumina powder and machined essentially by diamond polishing to make them at the dimensions of the housing in which they are currently secured by being driven therein. The method according to the invention consists however, in using a setting device very precisely, with respect to a reference surface of the part wedged onto the setting device, the spatial position of the jewel with respect to its housing whose machining tolerances will therefore be able to be much less strict.
The method consists in:
a) providing the setting device with spatial positioning means for one or several jewels with respect to one or several reference planes of said setting device and means for wedging a part,
b) wedging the part in which the jewel is to be set on the setting device,
c) putting one or several jewels in the respective housings securing their spatial position in the part with the positioning means of the setting device,
d) focusing and moving a laser beam tip on the points of abutment of the jewel against the wall of a housing, on the periphery of the opening of a housing or on a fusible material added around said opening so as to secure the jewel to the housing, and
e) releasing and removing the part from the setting device.
When the jewels are intended for pivoting wheels, and in particular when they are jewel holes, otherwise designated xe2x80x9cbearingsxe2x80x9d, the positioning means are formed by studs anchored in the surface of the setting device and passing through a through passage provided at the bottom of the housing. The diameter of the distal part of the stud is substantially equal to the diameter of the hole of the bearing. The part anchored in the setting device may have the same diameter than the distal part, the jewel resting on the bottom of the housing which then will have to be machined to a very precise dimension. The part of the stud anchored can also have a larger diameter than the distal part allowing a shoulder to be provided for vertically positioning the jewel with respect to the reference surface of the part, independently of the accuracy of manufacture of the bottom.
For prismatic jewels, such as escapement pallets or escapement impulse-pins, the setting device includes positioning means which are reversed with respect to those previously cited, namely cavities arranged in the surface of the setting device and in which the jewels can be temporarily immobilised with great precision by locking means, such as locking screws or spring pins.
Depending on the shape of the parts in which the jewels are to be set, it may be necessary to provide a setting device with several levels.
Of course, the spatial positioning accuracy of the jewels also depends on the wedging accuracy of the jewel, such accuracy can be obtained for example by means of stops arranged on an edge of the part, or by means of studs passing through it, these wedging means being mounted with precision on the setting device.
As will be indicated in more detail in the following examples, once the jewel has been positioned, it is definitively secured in the housing by means of a laser beam allowing the jewel to be welded to the wall, or the small play existing between the outer edge of the jewel and the wall to be filled either by melting an edge above the opening, in a single piece with the part, or by melting an added fusible material.